Joint replacement surgery to remove and replace arthritic or dysfunctional joint surfaces with orthopedic prosthesis is widespread today. Hip and knee replacement operations are among the most commonly performed operations in the U.S. Knee replacement surgery can involve one or more damaged knee surfaces. Total knee arthroplasty (“TKA”) is used to treat various forms of knee arthropathies, and is performed by removing and replacing all damaged articulating surfaces of the knee with a prosthetic component. Generally, TKA includes one or more of the following steps: a) removal of damaged cartilage along with a small amount of bone from the tibia and femur, b) replacement of damaged bone surfaces with metal components, and replacement of cartilage with medical grade plastic, c) resurfacing of the underside of the patella with a plastic disk, and d) inserting a piece of smooth, medical grade plastic between the metal parts to facilitate ease of movement.
Many different types of articulating material combinations are used to make prosthetic components. One of the more commonly used combinations includes a metal femoral surface articulating against a polymer tibia and patella surface. After implantation, the friction these components are subjected to may lead to metal erosion. Metal ions and debris released at the area of the implant can lead to complications such as osteolysis. Build-up of metal debris in soft body tissue can cause metallosis. Although poisoning from metallosis is rare, metal-on-metal implants containing cobalt-chromium alloys are known to cause arthroprosthetic cobaltism, which is an established health condition.
Stress shielding by stiff metal prosthetic implants can lead to bone resorption. Stress shielding is a mechanical phenomenon that occurs in prosthetic composites of stiff and flexible materials. In its natural state, a femur carries load by itself. However, when provided with a stiff metal implant, the femur is subject to reduced stress, hence stress shielded. Clinical patient series and animal experiments have shown that reduce stress loading can lead to bone loss. Changes in bone morphology have been linked to the effect of stress shielding and a subsequent adaptive remodeling process.
Stiff metal prosthesis are generally implanted to bone by long anchor pins and secured by using bone cement such as polymethylmethacrylate compositions, for example. Aseptic loosening of the bone cement surface, i.e., failure of the bond between an implant and bone in the absence of infection, has been observed in many cases.